Composite alumina/metal powders, cermets made from said powders, and processes of production

ABSTRACT

The invention relates to nano-composite powders of alumina and metal constituted of grains of micronic size. Each grain comprises a compact matrix of alumina of a specific surface area less than 5 m 2  /g, in which are dispersed crystallites of transition metals of alloys of these metals, of sizes less than 50 nm. The powder according to the invention may be produced starting with a precursor comprised of a mixed carboxylic salt of aluminum and one or more transition metals. The powders according to the invention permit producing by sintering cermets of alumina/metal benefitting from greatly improved mechanical and thermo-mechanical properties.

This invention relates to composite powders of ceramic/metal of the typecomprising a matrix of alumina and at least one transition metal, inparticular iron, chromium, molybdenum, cobalt, nickel or niobium, whichis present in the form of a dispersion of fine particles on the interiorof each grain of the matrix. The invention also relates to powderscalled "nano-composites" in which the metal particles are on the scaleof a nanometer. The invention also relates to cermets obtained bysintering said powders and to processes for production of these powdersand cermets.

BACKGROUND AND OBJECTS OF THE INVENTION

Composite alumina/metal powders are used for producing catalysts, or forfabrication by sintering cermets. The latter have numerous applicationsin various sectors of industry by reason of their thermo-mechanical anddielectric properties (by the term "cermet" is customarily meant acomposite material mass of ceramic/metal).

Composite alumina/metal powders presently known are essentially of twotypes:

microcrystalline powders of high porosity for production of catalystsupports (J. J. Chen, E. Ruckenstein, Journal of Catalysts 69, (1981),254-273; A. Uneo, H. Suzuki, Y. Kotera, Journal of the Chemical Society,Faraday Transaction 79, (1983), 127-136),

amorphous gels (L. Ganapathi et al, Journ. Solid State Ch., vol. 66,1987, pages 376-378; J. T. Klomp et al, Ceramurgia int., vol. 4, 1978pages 59-65; D Chakravorty, Sadhana, vol. 13, 1988, pages 13-18 . . . ).

These powders are comprised of matrices of γ or amorphous alumina and ametallic dispersion. Their main drawback is in being thermally unstablesuch that they do not permit production by sintering of cermets havinggood mechanical properties. During sintering, the metal particles have,in effect, a tendency to coalesce and to migrate toward their grainjunctions leading to a heterogeneous dispersion in which the metal phaseis found in the form of particles of large size juxtaposed with grainsof ceramic, with appearance of porosities between these phases. Thismicrostructure leads to poor properties in a mechanical sense, and athermal instability arising from differences of the coefficients ofexpansion of the phases (which causes a rupturing of the structureduring heat treatment).

Certain prior art documents have suggested the possibility of mixing αalumina with a metal binder (patent DD 137,313; U.S. Pat. No. 4,397,963,"Proceeding of the 21st automotive technology coordination meeting, "March 1984 Society of Automotive Engineers Inc., Warrendale, Pa., USA;American Ceramic Society Bulletin, vol. 61, no. 9, September, 1982,Columbus US pages 974-981, C. S. Morgan, et al: Thermal-Shock ResistantAlumina-metal Cermet Insulators"). However, in the powders thusobtained, the metal is arranged on the periphery of the grains: thesepowders have a microstructure which is fundamentally different from thatprovided by the present invention, since the metal is not inserted onthe interior of each grain of the matrix. The sintering of these powdersleads to microstructures of cermets similar to those indicated abovewith inherent drawbacks.

The present invention proposes providing new alumina/metal compositepowders in which the metal is present in the form of a dispersion on theinterior of each grain of the matrix, and their process of production.The invention seeks to overcome the drawbacks of known powders andpermits the production of cermets benefitting from greatly improvedmechanical properties and a good thermo-mechanical relationship.

In particular, the invention seeks to permit the production ofalumina/metal cermets which are apt to sustain thermal shocks.

Another object of the invention is to permit production of powders fromseveral metals, in which the metal dispersion is in the form of an alloyin order to benefit from the properties of the latter.

DESCRIPTION OF THE INVENTION

To this end, the composite ceramic/metal powder provided by theinvention, which may be obtained by the process defined below, isconstituted by micron-size grains comprising alumina and at least onetransition metal, and is characterized in that:

each grain comprises a compact matrix, of a specific surface area lessthan 5 m² /g,

said matrix is comprised of α alumina (corundum) of hexagonal structure,

the transition metal or metals are dispersed in each grain at the centerof the alumina matrix in the form of small crystals of sizes less than50 nanometers (designated below as "nano-crystals"),

the weight ratio of metal/alumina is less than 30%.

The microstructure of such a powder is fundamentally different from thatheretofore produced with α alumina, since the metal or metals arepresent in the form of very fine particles dispersed on the interior ofeach grain of alumina (and not a metal coating arranged around eachgrain of alumina or of particles arranged between the grains ofalumina). The powder according to the invention is freed of problems ofwettability of the alumina by the transition metals (problems which inknown powders are the source of the phenomena of coalescence andsegregation of the metals during heat treatments) due to a homogeneousdispersion of metal nanocrystals in the non-porous micronic matrices ofα alumina (this phase being thermally stable).

The composite powder conforming to the invention permits producingcermets comprising a ceramic matrix of α alumina in which are dispersed,in an intragranular manner, metal particles of sizes less than 100nanometers.

These cermets are obtained by sintering the powders, in particular underthe following conditions:

a minor addition to the powder of an organic binder having adecomposition temperature of between 150° C. and 300° C., decompositionbeing accompanied by a release of CO,

compacting of the powder/binder mixture,

heating of the compacted mixture, under a neutral atmosphere or reducedpressure, at a temperature of between 1350° C. and 1550° C.

Observations of the cermets thus obtained, carried out with an electronmicroscope and by X-ray diffraction, permit establishing very lowcoalescence of metal crystals of which the size increases slightlyduring the sintering, but which remain captive by ceramic matrices inthe form of small intragranular particles, with a slight percentage ofintergranular particles of which the size remains low (less than 100nanometers.) Such cermets have a compact structure with a rate ofdensification greater than 98%, which leads to excellent mechanicalproperties, in particular:

a resistance to bending comprising between 500 and 1000 megapascals,

a resilience comprising between 5 and 10 megapascals per √m.

Moreover, tests have shown that these cermets subjected to heattreating/tempering cycles (heating to 600° C. followed by tempering inwater) do not cause any damage, even at 30 cycles, while known cermetsburst at the end of about ten cycles, and alumina alone withstands onlya few cycles. These exceptional properties of thermal stability comefrom the structure itself of the cermet obtained, in which the α aluminais reinforced by the very fine intragranular dispersion. In effect, theplastic deformation of the metal phase permits absorbing all or part ofthe differential elastic deformations induced by thermal shock. In otherwords, the tenacity of such cermets is higher than that of pure alumina,a part of the energy of propagation of cracks being absorbed by metalparticles.

The ceramic/metal composite powder according to the invention, may inparticular be produced by the process defined hereafter, whichcomprises:

(a) preparing an aqueous solution of a mixed carboxylic salt of aluminumand one or several transition metals, of the formula Al_(1-x) M_(x)(R)_(n) where M represents the transition metal(s), R is a carboxylicradical, x is less than 0.3 and n is a whole number,

(b) precipitating this mixed salt by an organic solvent miscible withwater, in which said salt is stable and insoluble,

(c) separating the precipitate obtained from the liquid phase andrecovering the precipitate in the form of a micronic powder of mixedsalt, called a precursor,

(d) subjecting said precursor to a heat decomposition treatment in thepresence of oxygen at a temperature between 300° C. and 500° C. underconditions appropriate for decomposing the precursor and producing amixed amorphous oxide of aluminum and the transition metal(s) [Al₂ O₃].sub.(1-x) M_(2x) O_(y) where y is a whole number which is a functionof the valence of the transition metals,

(e) in the case of metals or alloys having a fusion point lower than1600° C., subjecting the mixed oxide to a thermal treatment by reheatingin the presence of oxygen to a temperature between 1000° C. and 1300° C.in order to obtain a crystallized solid solution of aluminum and anoxide of the transition metal or metals,

(f) reducing either the amorphous mixed oxide from step (d), or in thein the case of metals or alloys of low fusion point, the solidcrystalline solution from step (e), by a heat treatment in a reducingatmosphere free of water vapor at a temperature of between 1000° C. and1300° C. for a period greater than 2 hours.

Obtaining the aforementioned characteristics of the powder(nanocrystalline metals dispersed in micronic matrices; compact natureof these matrices; the type of alumina obtained) is essentially achievedby:

the use of a mixed precursor,

the precipitation conditions thereof,

the decomposition conditions of the precursor,

the heat treatment conditions of the decomposition residues.

The powders obtained by carrying out the above defined process have beenanalyzed by X-ray diffraction and have been observed by electronmicroscopy. In most of these cases, the transition metal or metals aredispersed in the aluminum matrix of each grain with a distribution ofsizes such that 90% in number of the metal particles have sizes arrangedover an interval less than 8 nanometers, and most often between 1 and 7nanometers.

The process of the invention permits in particular producing compositepowders comprised of grains having an aluminum matrix and at least onemetal of the following group: iron, chromium, molybdenum, cobalt,nickel, and niobium. It is sufficient to prepare the aqueous solution ofthe mixed carboxylic salt (a) from at least one salt of thecorresponding metal.

The process of the invention permits also the production of compositepowders constituted of grains comprising an alumina matrix and at leasttwo transition metals, dispersed in the matrix in the form of a metalalloy. The alloyed form of the metals of crystallites has beenestablished by X-rays, electron microscopy and by X energy dispersion.It is sufficient for producing such powders, to prepare the aqueoussolution of mixed carboxylic salt (a) starting from at least two saltsof metals able to form an alloy, particularly iron/chromium,nickel/cobalt, nickel/chromium.

According to a preferred embodiment, (a) the solution of mixed salt isprepared by mixing in an aqueous medium, oxalic acid or a salt of oxalicacid a salt of aluminum, and at least one salt of a transition metal, inorder to produce the mixed carboxylic salt by a complexing reactionbetween the oxalic radicals, the metal aluminum ions and the ions of thetransition metal or metals. In particular, one can choose betweenammonium oxalate, chloride or nitrate of aluminum and the chloride ornitrate of the transition metal or metals, in order to form thefollowing mixed carboxylic salt:

    Al.sub.(1-x) M.sub.x (C.sub.2 O.sub.4).sub.3 (NH.sub.4).sub.3

To further improve the purity and morphology of the powders obtained(regularity of the shape and size of the matrices), the processaccording to the invention may be carried out under the followingconditions:

(a) an aqueous solution of a mixed carboxylic salt having aconcentration between 0.1 and 3 moles/l is prepared,

(b) the mixed carboxylic salt is caused to precipitate while adding theaqueous solution into a alcohol solvent or a mixture of an alcoholicsolvent and another organic solvent, or a mixture of alcoholic solvents,in particular a mixture of ethanol/ethylene glycol or methanol/ethyleneglycol,

the aqueous solution of the mixed carboxylic solvent is poured into thesolvent in such a manner that the volumetric ratio between said aqueoussolution and said solvent comprises between 5 and 20, the medium beingagitated for a period of time at least equal to 30 minutes at atemperature of at most 30° C.,

(c) the precipitate is separated by filtration or centrifugation, and iswashed with acetone or ethanol, and is dried at a temperature below 80°C.,

(d) the decomposition treatment is carried out while slowly heating thepowder in a flow of air, with a speed of temperature rise at most equalto 2° C. per minute, to a floor temperature preferably of between 370°C. and 450° C., and maintaining the powder at this floor temperature forat least one hour.

In the case of metals or alloys having a low melting point (less than1600° C.), the reheating treatment which follows reduces the porosity ofthe mixed alumina oxides and, as a result, limits the coalescingphenomena during the step following the reduction (the metal atoms have,in effect in this case, a very high tendency to diffusion by reason ofthe low difference between the reduction temperature and the meltingpoint: the compactness accruing from the alumina matrices limits thistendency.)

For the following metals or alloys (with low melting points): iron,cobalt, nickel, iron/chromium, cobalt/nickel, nickel/chrome, (e) thisreheating treatment of the mixed oxide is preferably achieved during aperiod at least equal to 30 minutes, (f) the reduction of the solidcrystallized solution being then carried out under an atmosphere of dryhydrogen for a period of time between 2 and 20 hours.

In the case of high melting point transition metals such as chromium andniobium, (f) the reduction of the mixed amorphous oxide from step (d) isdirectly acted upon under a dry hydrogen atmosphere for a period ofbetween 10 and 20 hours. This period of time permits a goodcrystallization of α alumina and eliminates all porosity.

In the case of molybdenum, (f) the reduction of the mixed amorphousoxide from step (d) is directly carried out in an atmosphere of dryhydrogen while heating the oxide initially to a floor temperaturebetween 400 and 500° C. for a period of time between 1 and 5 hours, thento a final temperature between 1000° and 1200° C. for a period ofbetween 5 and 20 hours. This reduction in two successive stages avoidsall risk of sublimation of the molybdenum oxides when one reaches thetemperature of 800° C.

DESCRIPTION OF THE DRAWINGS

The invention described above in its general form, is illustrated byExamples 1 to 10 which follow, with reference to the accompanyingdrawings. In these drawings:

FIGS. 1, 3, 5, 7 and 8 are diagrams of the size distribution of thecomposite powders obtained respectively in examples 1, 3, 5, 7 and 8; onthe abscissa is arranged a logarithmic scale of the average diameter ofeach granulometric class (in microns), and on the ordinate thevolumetric percentage of these grains,

FIGS. 2, 4, 6 and 9 are graphs of the distribution of sizes of metalparticles dispersed in the composite powders obtained respectively inexamples 1, 3, 5 and 8. On the abscissa is shown the size of the metalparticles in nanometers and on the ordinate their percentage by number(from a count of 1000 individual grains measured from micrographsobtained from the electron transmission microscope);

FIGS. 10, 12, 14, 16 and 17 are micrographs of composite powdersobtained respectively in examples 1, 3, 5, 7 and 8;

FIGS. 11, 13, 15 and 18 are micrographs of cermets obtained respectivelyin examples 2, 4, 6 and 9 (it should be noted that to facilitateobservation, the cermets corresponding to the micrographs 13, 15 and 18have been metallized with gold).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Example 1

Preparation of a composite alumina/iron powder containing 5.4% by weightof iron.

a) A solution A is prepared from:

53.46 g of aluminum nitrate (Al(NO₃)₃.9H₂ O),

3.03 g of iron nitrate (Fe(NO₃)₃.9H₂ O),

63.95 g of ammonium oxalate ((NH₄)₂ C₂ O₄.H₂ O),

150 cm³ of distilled water.

Solution A is agitated for 40 minutes in order that the followingcomplexing reaction is completed:

    0.95Al(NO.sub.3).sub.3 +0.05Fe(NO.sub.3).sub.3 +3(NH.sub.4).sub.2 C.sub.2 O.sub.4 →(NH.sub.4).sub.3 Al.sub.0.95 Fe.sub.0.05 (C.sub.2 O.sub.4).sub.3 +3NH.sub.4 NO.sub.3

The molar concentration of oxalate of this solution is 0.75 mole/l.

b) A solution B containing 750 cm³ of ethanol and 750 cm³ of ethyleneglycol is prepared with agitation. Solution A is added into solution Bwith agitation. The volumetric ratio between solution A and solution Bis equal to 7.5. The mixed oxalate (NH₄)₃ Al₀.95 Fe₀.05 (C₂ O₄)₃precipitates after several minutes The agitation is maintained for 1hour at ambient temperature (20° C.).

c) The precipitate is filtered, then washed with ethanol. It is thendried in the stove (70° C.), then disagglomerated by grinding andscreened. It is analyzed by chemical analysis and thermogravimetry, andis comprised of the mixed oxalate (NH₄)₃ Al₀.95 Fe₀.05 (C₂ O₄)₃.

d) The precursor thus obtained is decomposed in air in a furnace at 400°C., the speed of heating being 2° C./mn, the time of heating being onehour.

e) The decomposition residue is then brought to 1150° C. for 2 hours ata heating speed of 5° C./mn. Radiocrystallographic analysis revealedthat at this step, a solid solution of alumina and hematite is obtained.The chemical dosage enables us to conclude the presence of the phase αFe₀.1 Al₁.9 O₃.

f) The mixed oxide is reduced under dry hydrogen at a temperature of1050° C. for 3 hours. Radiocrystallographic analyses showed that thepowder obtained is composed of α alumina (hexagonal structure) andmetallic iron. Granulometric analyses (FIG. 1) indicates that theaverage size of the grains of alumina is 1.5 μm. The specific surfacearea of the composite powder, measured by the B.E.T. method, is 1.95 m²/g. Study of the dispersion of the metallic particles is carried out byelectron microscope in transmission. A system of analysis by X energydispersion (EDAX) is connected to the microscope and permits making achemical analysis of zones of 10 nm per beam. Microscopic studies showthat the metallic particles appear with a dark contrast (FIG. 10). Thedistribution graph of the iron particle sizes shows that the averagesize of the metallic particles is 2.6 nm (FIG. 2). 90% of the metallicparticles have sizes between 1 and 5 nanometers.

Example 2

Preparation of a cermet from the composite powder obtained in Example 1.

3 g of the alumina/iron powder obtained from the preceding Example 1were mixed with 0.5 g of polyvinylic acid. This mixture is pressed undera pressure of 43 MPa, under a vacuum, at a temperature of 1450° C. for15 minutes. The extent of densification of the cermet obtained was 99%.The microstructure of this cermet is studied by electron transmissionmicroscopy (FIG. 11). These microstructural characteristics and themechanical properties of the cermet are summarized in the summary tableprovided at the end of the description.

Example 3

Preparation of a composite alumina-iron powder containing 10.8% byweight of iron.

a) A solution C is prepared starting with:

50.64 g of aluminum nitrate (Al(NO₃)₃.9H₂ O),

6.06 g of iron nitrate (Fe(NO₃)₃.9H₂ O),

63.95 g of ammonium oxalate ((NH₄)₂ C₂ O₄.H₂ O),

150 cm³ of distilled water.

The solution C is agitated for 40 minutes in order that the followingcomplexing reaction is complete:

0.9 Al(NO₃)₃ +0.1 Fe(NO₃)₃ +3 (NH₄)₂ C₂ O₄ →(NH₄)₃ Al₀.9 Fe₀.1 (C₂ O₄)₃+3NH₄ NO₃

The molar concentration of the oxalate in this solution is 0.75 mole/l.

b) A solution D containing 1497 cm³ of ethanol and 3 cm³ of ammonia isprepared with agitation. The solution C is added into the solution Dwith agitation. The mixed oxalate (NH₄)₃ Al₀.9 Fe₀.1 (C₂ O₄)₃precipitates after several minutes. The agitation is continued for 1hour at ambient temperature (20° C.).

c-f) The conditions of filtration, drying, decomposition of the oxalicprecursor, heat treatments for reheating and reducing are similar tothose described in Example 1. The same analyses are carried out on thecomposite powder. The results are as follows:

specific surface area: 1.5 m² /g,

average size of the grains of powder (FIG. 3): 1.45 μm,

average size of iron particles: 4.5 nm (graph of FIG. 4 and micrographof FIG. 12),

90% of the metallic particles have sizes between 2 and 7 nanometers.

Example 4

Preparation of a cermet starting with the powder obtained in Example 3.

The composite powder is densified under the conditions described inExample 2. The extent of densification is 99%. The microstructure of thecermet thus obtained is presented in the micrograph of FIG. 13. Themicrostructural characteristics and the mechanical properties of thecermet are summarized in the summary table at the end of thedescription.

Example 5

Preparation of a composite aluminum iron/chromium alloy containing 10.8%by weight of alloy.

a) A solution E is prepared starting with:

50.64 g of aluminum nitrate (Al(NO₃)₃.9H₂ O),

4.85 g of iron nitrate (Fe(NO₃)₃.9H₂ O),

1.20 g of chromium nitrate (Cr(NO₃)₃.9H₂ O),

63.95 g of ammonium oxalate ((NH₄)₂ C₂ O₄.H₂ O),

150 cm³ of distilled water.

The solution E is agitated for 40 mn in order that the followingcomplexing reaction is complete:

    0.9 Al(NO.sub.3).sub.3 +0.8 Fe(NO.sub.3).sub.3 +0.02 Cr(NO.sub.3).sub.3 +3 NH.sub.4).sub.2 C.sub.2 O.sub.4 →(NH.sub.4).sub.3 Al.sub.0.9 Fe.sub.0.08 Cr.sub.0.02 (C.sub.2 O.sub.4).sub.3 +3NH.sub.4 NO.sub.3

The molar concentration of oxalate is equal to 0.75 mole/l.

b) A solution F containing 750 cm³ of methanol and 750 cm³ of ethyleneglycol is prepared with agitation. The solution E is added into thesolution F with agitation. The mixed oxalate (NH₄)₃ Al₀.9 Fe₀.08 Cr₀.02(C₂ O₄) precipitates after several minutes. The agitation is continuedfor 1 hour at ambient temperature (20° C.).

c-e) The conditions of filtration, drying, decomposition of the oxalicprecursor, the thermal and reheating treatments are similar to thosedescribed in Example 1.

f) The mixed oxide Al₁.8 Cr₀.04 Fe₀.16 O₃ thus synthesized is reducedfor 10 hours under dry hydrogen at 1050° C. The same analyses aspreviously were carried out on the composite powder. The results are asfollows:

specific surface area: 1.9 m² /g,

average size of grains of powder (FIG. 5): 1.3 μm,

average size of particles of the alloy Fe₀.8 Cr₀.2 : 4.0 nm (graph ofFIG. 6 and micrograph of FIG. 14.)

Example 6

Preparation of a cermet from the powder obtained in Example 5.

The composite powder is densified under the conditions described inExample 1. The microstructure of the cermet thus obtained is shown inthe micrograph of FIG. 15. The microstructural characteristics andmechanical properties of the cermet are summarized in the summary tableat the end of the description.

Example 7

Preparation of an alumina-molybdenum powder containing 10.8% by weightof molybdenum.

a) A solution G is prepared starting with:

50.64 g of aluminum nitrate (Al(NO₃)₃.9H₂ O),

2.40 g of molybdic acid (NH₄)₂ Mo₄ O₁₃),

63.95 g of ammonium oxalate ((NH₄)₂ C₂ O₄.H₂ O),

150 cm³ of distilled water.

The precursor (NH₄)₃.14 Al₀.9 (MoO₃)₀.1 (C₂ O₄)₃ is obtained.

b) A solution B (identical to that prepared in Example 1) containing 750cm³ of ethanol and 750 cm³ ethyleneglycol is prepared with agitation.The solution G is added into the solution B with agitation. The mixedoxalate (NH₄)₃.15 Al₀.9 (MoO₃)₀.1 (C₂ O₄)₃ precipitates after severalminutes. The agitation is maintained for 1 hour at ambient temperature(20° C.).

c-d) The conditions of filtration, drying, decomposition of the oxalicprecursor are similar to those described in Example 1.

f) The mixed amorphous oxide thus obtained is directly treated under dryhydrogen, without annealing because of the high melting point of themolybdenum (1610° C.). The reduction process is carried out in thefollowing manner: speed of heating 5° C./mn, 450° C. plateau for 2hours, 1150° C. plateau for 5 hours. The same analyses as before arecarried out on the composite powder. The results are as follows:

specific surface area: 2 m² /g,

average size of grains of powder (FIG. 7): 1.3 μm,

average size of molybdenum particles: 40 nm (micrograph of FIG. 16).

Example 8 Preparation of aluminum-chromium powder containing 21% byweight of chromium.

a) A solution H is prepared starting with:

45.02 g of aluminum nitrate (Al(NO₃)₃.9H₂ O),

12.0 g of chromium nitrate (Cr(NO₃)₃.9H₂ O),

63.95 g of ammonium oxalate ((NH₄)₂ C₂ O₄.H₂ O),

150 cm³ of distilled water.

The solution H is agitated for 40 minutes in order that the followingcomplexing reaction is completed:

    0.8 Al(NO.sub.3).sub.3 +0.2 Cr(NO.sub.3).sub.3 +3(NH.sub.4).sub.2 C.sub.2 O.sub.4 →(NH.sub.4).sub.3 Al.sub.0.8 Cr.sub.0.2 (C.sub.2 O.sub.4).sub.3 +3NH.sub.4 NO.sub.3

The molar concentration of oxalate is 0.75 mole/l.

b) A solution B (identical to the one of Example 1) containing 750 cm³of ethanol and 750 cm³ of ethylene glycol is prepared with agitation.The solution H is added into the solution B with agitation. The mixedoxalate (NH₄)₃ Al₀.8 Cr₀.2 (C₂ O₄)₃ precipitates after several minutes.The agitation is continued for 1 hour at ambient temperature (20° C.).

c-d) The conditions of filtration, drying, decomposition of the oxalicprecursor are similar to those described in Example 1.

f) The mixed amorphous oxide Al₁.6 Cr₀.4 O₃ thus synthesized is directlyreduced over 20 hours under hydrogen at 1050° C. (melting temperature ofchromium: 1875° C.). The same analyses as before are carried out on thecomposite powder. The results are as follows:

specific surface area: 1.9 m² /g

average size of grains of powder (FIG. 8): 1.3 μm,

average size of particles of chromium: 4.0 nm (diagram of FIG. 9 andmicrograph of FIG. 17).

Example 9

Preparation of a cermet starting with the powder obtained in Example 8.

The composite powder is densified under the conditions described inExample 1. The degree of densification is 99%. The microstructure of thecermet thus obtained is shown in the micrograph of FIG. 18. Themicrostructural characteristics and mechanical properties of the cermetare compiled in the summary table which follows:

This table summarizes the structural and mechanical properties of thecermets obtained in Examples 2, 4, 6 and 9 described above:

                  SUMMARY TABLE                                                   ______________________________________                                                                         K.sub.ic                                     Example C (%)   d (nm)   σ.sub.f (MPa)                                                                   (MPa/√ m)                                                                        N                                  ______________________________________                                        2       5.4     20       600     7.2       50                                 4       10.8    30       530     6.8       30                                 6       10.8    20       650     7.5       27                                 9       21.0    25       600     6.5       30                                 ______________________________________                                         C: weight percent of metallic phase                                           d: average diameter of metallic particles (RX, Scherrer method)               σ.sub.f : resistance to rupture in three point flexure                  K.sub.ic : Factor of the intensity of the critical constraint ("S.E.N.B."     method), toughness                                                            N: number of thermal shocks to rupture by plunging into water                 (Δ.sub.T = 600° C.)                                         

By way of comparison, given hereafter are the characteristics of aceramic of alumina αAl₂ O₃ sintered under the same conditions (degree ofdensification 99%):

σ_(f) =450 MPa

K_(ic) =4 MPa/√m

N=3 cycles

Example 10

Preparation of a composite alumina-iron/chromium alloy powder containing21% by weight of the alloy

a) A solution is prepared starting from:

45.02 g of aluminum nitrate (Al(NO₃)₃.9H₂ O),

6.06 g of ferric nitrate (Fe(NO₃)₃.9H₂ O),

6.00 g of chromium nitrate (Cr(NO₃)₃.9H₂ O),

63.95 g of ammonium oxalate ((NH₄)₂ C₂ O₄.H₂ O),

150 cm³ of distilled water.

The solution is agitated for 1 hour in order that the followingcomplexing reaction is complete:

    0.8 Al(NO.sub.3).sub.3 +0.1 Fe(NO.sub.3).sub.3 +0.1 Cr(NO.sub.3).sub.3 +3 (NH.sub.4).sub.2 C.sub.2 O.sub.4 →(NH.sub.4).sub.3 Al.sub.0.8 Fe.sub.0.1 Cr.sub.0.1 (C.sub.2 O.sub.4).sub.3 +3NH.sub.4 NO.sub.3

The molar concentration of oxalate is equal to 0.75 mole/l.

b-e) The conditions of precipitation, filtration, drying, decompositionof the oxalic precursor, thermal annealing treatment are similar tothose described in Example 1.

f) The mixed oxide Al₁.6 Cr₀.2 Fe₀.2 O₃ thus synthesized is reduced for20 hours under dry hydrogen at 1050° C.

The same analyses as before were carried out on the composite powder.The results are as follows:

specific surface area: 3.1 m² /g,

average size of the grains of powder: 2.2 μm,

average size of particles of the alloy Fe₀.5 Cr₀.5 : 4.2 nm.

We claim:
 1. A process for the production of a composite powder ofceramic/metal comprising:(a) preparing an aqueous solution of a mixedcarboxylic salt of aluminum and one or more transition metals, havingthe formula Al_(1-x) M_(x) (R)_(n) where M represents the transitionmetal or metals, R is a carboxylic radical, x is less than 0.3 and n isa whole number, (b) precipitating the mixed salt by an organic solventmiscible with water, in which said salt is stable and insoluble, (c)separating the precipitate obtained from the liquid phase and recoveringthe precipitate in the form of a micronic powder of mixed saltprecursor, (d) subjecting said precursor to a thermal decompositiontreatment in the presence of oxygen at a temperature of between 300° C.and 500° C. for decomposing the precursor and producing a mixedamorphous oxide of aluminum and the transition metal or metals (Al₂O₃)_(1-x)) M_(2x) O_(y), where y is a whole number which is a functionof the valence of the transition metal or metals, (e) in the case ofmetals or alloys having a melting point less than 1600° C., subjectingthe mixed oxide to an annealing heat treatment in the presence of oxygenat a temperature of between 1000° C. and 1300° C. for obtaining a solidcrystallized solution of aluminum and oxides of the transition metal ormetals, (f) reducing either the mixed amorphous oxide from step (d), orin the case of metals or alloys having a low melting point, the solidcrystalline solution from step (e), by a heat treatment under a reducingatmosphere free of water vapor at a temperature of between 1000° C. and1300° C. for a period of more than 2 hours.
 2. A process for productionas in claim 1 including preparing the aqueous solution of the mixedcarboxylic salt from at least one salt of a metal selected from thegroup consisting of iron, chromium, molybdenum, cobalt, nickel, niobiumor with at least two salts of these metals able to form an alloy.
 3. Aprocess as in claim 1, and including preparing the solution by mixing inan aqueous medium the oxalic acid or a salt of oxalic acid, a salt ofaluminum and at least one salt of a transition metal in order forproducing the mixed carboxylic salt by a complexing reaction between theoxalic radicals, the ions of metallic aluminum and the ions of thetransition metal or metals.
 4. A process as in claim 3, and includingchoosing the aluminum oxalate, chloride or nitrate of aluminum and thechloride or nitrate of the transition metal or metals, for forming thefollowing mixed carboxylic salt:

    Al.sub.(1-x) M.sub.x (C.sub.2 O.sub.4).sub.3 (NH.sub.4).sub.3.


5. A process as in claim 1, and wherein said aqueous solution of mixedcarboxylic salt has a concentration of between 0.1 and 3 moles/l.
 6. Aprocess as in claim 1, and including causing the mixed carboxylic saltto precipitate while adding the aqueous solution into an alcoholicsolvent or a mixture of an alcoholic solvent and another organicsolvent, or a mixture of ethanol/ethyleneglycol ormethanol/ethyleneglycol.
 7. A process as in claim 6 for the productionof a composite powder of alumina/iron, comprising using as the solventin step b ethanol with the addition of a minor amount of a base.
 8. Aprocess as in claim 5, and wherein step (b) comprises pouring theaqueous solution of mixed carboxylic salt into the solvent in such amanner that the volumetric ratio between said aqueous solution and saidsolvent comprises between 5 and 20, the medium being agitated for aperiod of at least 30 minutes at a temperature at most equal to 30° C.9. A production process as in claim 1, and wherein step (c) comprisesseparating the precipitate by filtration or centrifugation, washing theprecipitate with acetone or ethanol, and drying the precipitate at atemperature less than 80° C.
 10. A process for production as in claim 1,and wherein the decomposition treatment according to step (d) comprisesslowly heating the powder under a sweeping of air, at a speed oftemperature rise at most equal to 2° C. per minute, to a plateautemperature between 370° and 450° C., and maintaining the powder at thisplateau temperature for at least one hour.
 11. A process as in claim 1for producing a composite powder containing a low melting pointtransition metal selected from the group consisting of iron, cobalt ornickel, or a low melting point alloy: iron/chromium, cobalt/nickel ornickel/chromium, and subjecting the mixed oxide obtained from step (d)to an annealing heat treatment in air for at least 30 minutes.
 12. Aproduction process as in claim 11, and wherein the reduction of thecrystallized solid solution according to step (f) is carried out underan atmosphere of dry hydrogen for a period of between 2 and 20 hours.13. A process as in claim 1 for the production of a composite powdercontaining a high melting point transition metal selected from chromiumand niobium, and wherein the reduction of the mixed amorphous oxide fromstep (d) is directly carried out under an atmosphere of dry hydrogen fora period of between 10 and 20 hours.
 14. A process as in claim 1 for theproduction of a composite alumina/molybdenum powder, and wherein thereduction of the mixed amorphous oxide from step (d) is directly carriedout under an atmosphere of dry hydrogen while heating the oxide first toa plateau temperature of between 400 and 500° C. for a period comprisingbetween 1 and 5 hours, then heating to a final plateau temperaturecomprising between 1000° and 1200° C. for a period comprising between 5and 20 hours.
 15. A production process for a cermet comprising preparinga composite powder of ceramic/metal by carrying out the process of claim1 and sintering this powder under the following conditions:adding aminor amount of an organic binder having a decomposition temperature ofbetween 150° C. and 300° C., said decomposition being accompanied by arelease of CO, compacting the mixture of powder/binder, heating thecompacted mixture, under a neutral atmosphere or under reduced pressure,at a temperature of between 1350° C. and 1550° C.